You can try reinstalling Adobe. If Word still thinks it "owns" pdf files
after that, you can use Windows Explorer to change the association back to
Adobe.
Overview... start Windows Explorer. Locate a .pdf file, right-click it, and
choose Properties from the menu. In the General tab, next to Opens with:,
click on Change. Locate Adobe (Reader or Acrobat, whichever you have/want to
be associated). Click on it. Here "Always use the selected program..." is
checked and grayed out. If it's not grayed out for you, you'll want to
enable it. Click OK. Back in the Properties dialog, click on Apply.